KartoMNT is a free software of height model digitalization.
This software allows you to create your own Digital Height Models (DHM) with a simple map. Then you'll be able to visualize DHMs with the KartoMNT 3D viewer.

Punt is a Open Source multi-language desktop application that allows the user to view the terrain of any world in 3D. Using open standards Punt can make use of data from the Internet combined with local data in common GIS-compatible formats.